1. Field of the Invention
The present invention relates generally to implantable cardiac defibrillation leads, and more particularly to a defibrillation lead having a coated cable conductor therein and to a method of manufacturing for such leads. An apparatus for manufacturing cables by selective laser removal of insulation is described.
2. Description of the Related Art
Implantable medical devices for treating irregular contractions of the heart with electrical stimuli are well known in the art. Some of the most common forms of such implantable devices are defibrillators and pacemakers.
Defibrillators are implantable medical devices used to treat fibrillation, a condition characterized by rapid, chaotic electrical and mechanical activity of the heart""s excitable myocardial tissue that results in an instantaneous cessation of blood flow from the heart. Defibrillation is a technique employed to terminate fibrillation by applying one or more high energy electrical pulses to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to restore the normal synchronized contraction of the total mass of tissue.
A pacemaker, or pacer, is an implantable medical device that delivers low energy electrical pulses to stimulate a patient""s heart to beat at a desired rate in instances where the heart itself is incapable of proper self-regulation. This occurs when the heart""s natural pacemaker, which causes the rhythmic electrical excitation of the heart and pumping of blood, malfunctions due to age or disease. Demand pacing is a process used to maintain normal beating of a heart having this condition.
Various types of leads for defibrillators and demand pacers have been suggested in the prior art. For example, large electrical patches sewn to the exterior surface of the heart have been used to deliver defibrillation pulses to the heart. Implantation of such patch electrodes requires opening of the patient""s chest during thoracic surgery. For pacing, pulses may be applied to the heart with the use of a pacer lead having an exposed metal surface, or demand pacer electrode, extending through a vein and into the heart.
Those involved in the medical arts recognized that prior art defibrillators required a high threshold level of energy for effective defibrillation, which limited the useful life-span of the devices and, more significantly, posed a significant risk of causing electrolysis of the blood and myocardial damage. It was realized that the defibrillation electrode configuration played an important role in the amount of energy needed to achieve successful defibrillation. This led to the development of transvenous defibrillation leads having long coil-shaped defibrillation electrodes for implantation into the right ventricle of the heart through a vein. For example, U.S. Pat. No. 4,922,927, the entire disclosure of which is incorporated herein by reference, discloses a defibrillation electrode made up of a plurality of separate wires wound side-by-side to form a tight coil. The coil was disposed upon an insulated tubular member and had a length sufficient to extend throughout the entire length of the ventricular chamber to provide sufficient electrode surface area for defibrillation.
Transvenous cardiac stimulation leads, such as the device of U.S. Pat. No. 4,922,927, were configured to also carry a demand pacing electrode. Thus, a single device implantable in one surgical procedure could provide defibrillation and pacing pulses for heart patients suffering from both irregular heart beat and, at times, cardiac fibrillation. This eliminated the need for multiple and complex surgical procedures to attach the prior art electrodes required for both types of treatments.
Another defibrillation electrode configuration for use with dual purpose transvenous leads is disclosed in U.S. Pat. Nos. 5,476,502 and 5,374,287 to Rubin, which are also incorporated herein by reference in their entireties. The xe2x80x9cRubinxe2x80x9d catheter included either a helical or lance shaped defibrillation electrode for delivering a defibrillation pulse directly to the interior of the septum of the patient""s heart. The length of the helix-shaped electrode to be screwed into the septum from the right ventricle, about 0.5 cm to 1.0 cm, was substantially shorter than the conventional coiled transvenous defibrillation electrodes.
Despite these developments there continues to be a need for a lead capable of providing both high voltage defibrillation and effective demand pacing with a smaller lead diameter to minimize obstruction in the veins leading to the heart. One such lead has been developed by some of the inventors herein and others. A commonly-assigned patent application has been filed entitled Endocardial Defibrillation Lead with Looped Cable Conductors, attorney docket no. ITM-609 US, the disclosure of which is incorporated herein by reference. This lead has a looped cable conductor for conducting high voltage defibrillating shocks to the heart and a coil conductor for conducting low voltage pacing pulses. These two conductors are carried in separate lumens within a lead body. Additional lumens may be provided for additional conductors if additional functions are desired. The looped cable conductor has insulation which must be accurately removed at selected locations.
We have invented a method of manufacturing an implantable defibrillation lead with an elongated, flexible lead body having folded cable conductors and an apparatus for performing this method. According to our invention, a jig supports an insulated cable conductor in a looped configuration, exposing selected portions of the conductor over a linear stripping area. A laser selectively removes insulation from the cable. The cable is then folded into a doubled configuration and inserted into a lead body of an endocardial lead.
In a preferred embodiment, there is provided an implantable endocardial defibrillation lead having a looped cable conductor for conducting at least high voltage defibrillation shocks. A coil electrode is connected to an elongated, flexible, electrically non-conductive lead body and is supplied with electrical power for delivering electrical shocks to the heart through a looped cable conductor that extends through the lead body and is associated with a power source.
Depending upon the desired application for the lead, the invention may also be used with a pacer and, thus, include any of a variety of pacer electrodes and sensors that are presently available or may become available. Such devices, if used, would be disposed upon the lead, insulated from the defibrillator electrode segments and electrically connected with a second electrical conductor that extends through the lead body and provides electrical power to the pacer electrode. The lead may also include a ground electrode disposed upon the lead a distance from the other electrodes to receive the pulses delivered to the heart tissue and transmit them back through a third electrical conductor extending through the lead. The coil electrode and looped cable conductor may also serve a dual function as a ground electrode and conductor.
The invention may also be adapted for fixation of the distal end of the lead to the heart to achieve selective positioning of the electrode or electrodes. A variety of currently available passive and active fixation mechanisms, or that may become available, may be used with the invention, such as tines or a fixation screw for securing the distal end of the lead within the heart.
Moreover, multiple coil defibrillation electrodes may be used, for example, for placement in the ventricle and in the superior vena cava of a patient. The defibrillation electrode may be energized through a single cable conductor, in which case insulation must be removed from more than one area of the conductor. Furthermore, if the defibrillation coil or coils are used both for defibrillation an as an anode for cardiac pacing, low and high voltage connections to the cardiac stimulator are usually provided. This would require multiple areas without insulation at the proximal end of the lead as well. Our method and apparatus accurately prepares a cable conductor for use in a defibrillation lead.
The apparatus of our invention comprises a jig for holding at least one insulated wire, the jig having a base plate, a linear stripping area on the plate, a first clasp mounted on the plate for holding a first end of the wire, at least two primary pins mounted on the plate on a first side of the linear stripping area, at least one secondary pin mounted on the plate on a second side of the linear stripping area, and a second clasp mounted on the plate for holding a second end of the wire; and a laser effective to remove insulation from the insulated wire, the laser being mounted to effectively remove insulation in the linear stripping area.
The characteristics and advantages of the present invention described above, as well as additional features and benefits, will be readily apparent to those skilled in the art upon reading the following detailed description and referring to the accompanying drawings.